The objective of the proposed study is to complete the follow-up of children with hypothyroidism diagnosed as the result of the neonatal screening program in New England from January 1, 1976 through December 31, 1980. Such children who received adequate treatment have IQs that do not differ from those of their controls. They progressed normally through the third grade, thus refuting the notion that even those hypothyroid children with normal IQs are unable to maintain normal progress. The specific aims of the proposed study are two in number. The first is to shown whether or not the children continue to progress normally as school becomes more intellectually demanding by following them into high school. Because no cohort of children with infantile hypothyroidism has been followed this far in school, the results will be o practical importance. Demonstration of normal school progress will prevent the children from becoming victims of a teacher whose attitude is based on low expectations of their ability to learn. In addition to the psychometric tests administered by they study personnel, school performance will be monitored by annual interviews with parents and reports from schools of grades and class standings. The second specific aim is to document physical growth and sexual maturation of hypothyroid children through puberty and beyond the period of peak growth velocity. Even hypothyroid children who were treated adequately but diagnosed clinically had slightly delayed pubertal development. There are no data for children with infantile hypothyroidism treated shortly after diagnosis by neonatal screening. The spectre of potential problems in this cohort has been raised by several events, two of which are the finding of an increase incidence of congenital anomalies and an increased incidence of secondary hypopituitarism in the older children. In addition, the follow-up of cohorts of children with galactosemia and PKU has revealed unexpected striking problems in sexual maturation and fertility. The latter observations, coupled with the high incidence of congenital anomalies in infantile hypothyroidism, may be an indication of potential problems and thus suggest the importance of surveillance of the hypothyroid cohort through puberty.